I can find very little information on
the internet relating to when, exactly, Hugh Hefner's Playboy Press
began printing books. Graham Holroyd, in his Price Guide, starts listing
printing dates with 1970. I have found numerous volumes, from mass market
sized books to oversized softcover issues, with copyright dates
that go back to about 1963. But I don't think Playboy became a serious
player in the book industry until later in the 60's decade. See two pages
of Playboy paperback covers on the BookScans website
HERE
(click the "NEXT" prompt at the bottom of that page for more).

In the mid-60's, Pocket Book
printed a half dozen books that included both their own colophon and
Playboy's on the covers. They all contained reprinted content from Playboy
magazine. I'm not sure if Pocket sought out that business, or whether
Playboy was simply outsourcing their book printing until they could gear
up to do it themselves.